Stihl saw won't idle whether hot or cold

[JDHupp]

This actually concerns a Stihl TS 400 cutoff saw, but I'm sure the list of possible causes would be similar to a chainsaw.

This saw starts with 2 pulls with choke on and then 2 pulls with choke off.  It runs OK with some throttle, but it will die right away if I try to let it idle, and that is the case if the saw is hot or cold.

There are two carb adjustment screws, L low speed (coarser) adjustment and LA idle speed (finer) adjustment, but I find that even the main L screw adjustment has no effect on the problem.  Standard setting is one turn out, but I can go to three turns without touching idle performance.

A second problem is that acceleration is sluggish.

I'm in the process of replacing a slightly cracked fuel hose (the tank to carb hose, not the carb to cylinder hose), but I'm not sure the cracks are enough to cause this problem.

If replacing the fuel hose doesn't fix it, what are my other likely suspects?

The spark plug is almost new and looks clean.  And I have just rebuilt the carburetor (though I did not soak it in carb cleaner, just flushed with gas and installed the kit parts), replaced the in-tank fuel filter, and replaced the missing auxiliary air filter (and blown out the other two air filters) and missing spark arresting screen in the muffler.  Prior to doing those things, I could get the saw to idle for a while by setting the L screw a little higher than it was supposed to be, but it would eventually die anyway after idling erratically.  And it also had the poor acceleration at that point.  I was imagining that the idle problem got worse because I fiddled with the fuel hose some while removing/replacing the carb. 

[Kevin]

Could be a restricted fuel line,tank vent or fuel filter.
Leaking gasket between carb and cylinder.

[sawcollector]

A piston port two-stroke uses the piston skirt as the valve to open a close the intake port to the crankcase. If the intake side of the piston is worn, it no longer seals well and will cause idle problems. Cut-off machines are notorious for ingesting fine masonry dust which will wear away the intake skirt first, and eventually everything else in the engine. I hate to be so negative but just about every cut-off saw I see has pretty much ground itself to bits by the time it starts running bad enough to cause the operator to quit trying to use it. Take off the carb and look at the piston with a light and it should not be a dull grey, it should be aluminum colored and if in really good shape you can still see the machine marks from the turning process across the skirt. If it is real worn it may run so-so at full throttle but stumble and die at idle. To really know for sure take off the muffler and push on the bottom of the intake skirt with the eraser end of a pencil and push on the other side of the piston through the ex port with your finger and the piston should not rock back and forth. I had a TS 350 S that was so bad you hear the piston "clacking" back and forth when you pulled it through, but it still had compression and would run full throttle.
On the other hand, it may be the carb. One circuit feeds fuel at full throttle, and on the TS 400 the H side is fixed and the fuel is fed through a jet. The low side I think has a feed hole to flow fuel past the L screw and then under the large welch plug to flow the fuel through the idle transition holes. It is possible that the low side is restricted so no fuel delivery at idle but the H side is OK so it will run wide open. Take the carb apart and remove the L screw and use the little red tube on a can of brake cleaner spray and spray through the hole the L screw came out of, while looking into the venturi from the engine side of the carb, with the throttle open. You should see the spray come out of two or three little holes. Then look on the metering side and you should see it spray out of the feed hole.
Hope that helps...
Eddie

[Warbird]

sawcollector, that was great info.  You have provided awesome information to folks on a few occasions.  I've learned a ton from reading your posts.  Thanks!

[joe_indi]

JDHupp ,
The older TS400 had a  carb with H, L  and LA screws. hey were easy to adjust.
This new carb with the BIS (Bye-pass Induction System) is a real pain.
I know, 'cos I have the same problem.
Let me  mention how I have sorted out the headaches in my own fashion.
First, check  the condition of your air filter and pickup body.
Also check whether there is any dust accumulation
on top of the metering diaphragm through the air compensator.
Cold Starts
If you stick to using only the choke, you will use up all your energy before you get it to start.I remove the air filter cover and splash some fuel into the air intake.The Choke lever is moved to semi-choke and the engine starts on the first or second pull.

Engine dies off after finishing a cut or when hand is removed from the throttle
After starting the engine warm it up.Turn the LA screw inwards till you get a fairly fast idle.Now, reduce the idle by turning out the L Screw.

Joe

[sawcollector]

Thanks for the nice comments Warbird, appreciate it. Been at this a long time and I am a teacher by trade so sometimes I can't help myself!
Joe, please allow me to point out something. BIS stands for Balanced Idle System, and was never actually used on the TS 400. It is currently used on the little MS 170, 180, and was also used on some line trimmers and other power tools in the past. It is a fixed jet carb with no mixture screws for the H or L, and has a single idle screw that sets the idle RPM and L side mixture at the same time. It is a good design and works well, just does not let you adjust. Not good though if you plan to mod the mufler on your MS 170!
You are right though, that older versions of the TS 400 had a full adjust carb, and I agree that it is always nice to be able to tune the carb to the engine, for altitude, wear and tear, and application. The EPA changed all that for us when the mandate for lower emissions came out in 1996. I have to agree that it is good to lower emissions but some methods make it tough on the end user. Thankfully STIHL has gone back to adjustable carbs (with limit caps) on most current applications and probably all future models as well.
What was changed on the TS carb was that the H screw was removed and a fixed orifice jet now controls all the fuel flow at WOT. But the low side is still adjustable, and you are correct Joe that the idle should be tuned for the correct RPM, about 2800, so the wheel doesn't turn, and the L screw can be used to adjust for smooth idle and set a little rich to aid in acceleration.
OK JD what have you found out? We have offered up tons of advice and info and now require your findings!!!

[Rockfarmer]

Thanks for the info sawcollector  ;)

JD, I hate to say it but we had a pile of chopsaws in the corner of our garage at one of the masonry companies I worked for. They just dont get the air filters cleaned enough, run lean and hard with no love whatsoever. Best of luck to you.

[Al_Smith]

Chop saws get abused period . I wouldn't be a bit surprised to find that carb full of concrete dust .

Many times on a chainsaw little itty bitty bits of wood fiber clogs the internal screen in the carb ,it gets right passed the fuel filter .If wood fiber can get through a filter ,concrete dust would have no problem at all .

As far as the piston being worn ,no doubt it is .However I've had saws apart where the piston rattled around like a bb in a box car but they still would idle and run ,not real well though .

If I were a betting man my money would be on a funky carb as being the problem .